This invention relates to a random access memory, and, more particularly, to such a memory that can be interrupted during readout without loss of the stored information.
Many military and some civilian systems require data storage capability in a random access memory that is nonvolatile and resistant to data loss due to disruptive events. A nonvolatile memory retains the stored data even when power is turned off for a period of time. Such nonvolatile memories are often based on physical changes that occur in a memory element when data is stored, which physical changes are not reversed or randomized upon the loss of power.
Ferroelectric materials have been successfully developed for use in nonvolatile memories. A ferroelectric material is one whose physical state changes upon the application of an electrical field, in a manner analogous with the change undergone by ferromagnetic materials to which a magnetic field is applied. The ferroelectric material has the advantages that its physical state can be controlled by the application of a voltage rather than a magnetic field, a measurable physical state is retained after a power loss, and small-size memory elements can be constructed by microelectronics fabrication techniques, which result in memory elements which consume little power.
Random access memories can be classified as destructive readout (DRO) and nondestructive readout (NDRO). In the destructive readout memory, the physical state of the memory unit is altered upon readout such that the stored data value is lost to the memory unless Further action is taken. To avoid this loss of information, the stored value must be refreshed after each readout, so that the stored data are available for a subsequent readout. In the absence of refreshing of the data, it is possible to read the data only once.
In many applications, it may be assumed that refreshing can reliably occur as a regular part of a readout/refresh cycle. Destructive readout memories may therefore be employed in such situations. In other applications, however, that assumption cannot be made because there may be a disruption experienced by the system in the brief period of time between the moment when the readout is accomplished and the moment when refresh would normally occur. Such disruptions can result due to, for example, power interruptions to civilian systems and nuclear events which may be experienced by military systems.
To date, ferroelectric destructive readout memories have been developed and applied. While useful for some applications, such memories are of limited value where the system must be resistant to the effects of disruptive events. A nondestructive readout (NDRO) ferroelectric memory would have application in situations where the memory state must be retained through extended power outages must also be stable through brief, multiple disruptions that may occur when the content of the memory is being read. Several nondestructive readout memory approaches have been proposed. However, none are based on or require the partial switching of the polarization.